Some postmenopausal women use hormone replacement therapy (HRT) to maintain bone. This treatment is, however, not acceptable to all women. An alternative approach is intake of foods rich in phytoestrogens - natural plant estrogens such as soybeans. The hypothesis is that soy protein protects against postmenopausal bone loss by slowing resorption and enhancing bone formation due to organ-selective estrogenic effects of the isoflavones genistein and daidzein. The applicant proposes four questions: 1. Efficacy: What is the effect of soy protein and soy isoflavones on bone formation and resorption? 2. Safety: Do soy isoflavones and estrogen have differential effects on bone and female reproductive tissues? 3. Safety: Do soy proteins or isoflavones antagonize the effects of endogenous estrogen on bone and the reproductive tract? 4. Mechanism: Do soy isoflavones differentially affect estrogen-dependent bone resorption and formation through estrogen receptor-mediated mechanisms? In vivo studies will use ovariectomized rats, an established model for osteoporosis, and intact rats. In vitro studies will use mouse calvarial explant culture and an estrogen receptor reporter system. To answer questions 1-3 ovariectomized and inteact rats will be fed diets of various concentrations of soy or isoflavones for 3 months. Measurements during and at termination of experiments are urinary deoxypyridine, an indicator of bone breakdown; bone mineral density; and bone formation by histomorphometry. The reproductive tract will be evaluated by histology and production of Complement C3 by the uterus and measurement of estradiol and luteinizing hormone in plasma. In vitro studies (question 4) will determine if isoflavones act through an ER and which subtypes, ERa or ERb. These studies are necessary to identify efficacious and safe alternative approaches to HRT readily accessible to all women. Knowledge of active agents and mechanism of action will allow standardization of soy supplements; identification of doses that maintain bone health without compromising other systems, e.g. reproductive; and assurance against interactions from like compounds in other drugs and food, including supplements.